The present invention relates to a method of forming an electrode for a flat display panel, and more specifically relates to a method of forming an electrode in a space enclosed by partition walls in a plasma display panel (PDP), etc.
FIG. 1 is a perspective view showing essential parts of a general surface discharge AC PDP. A PDP 1 is a self-emission type thin display panel constructed by disposing a front substrate 10 comprising glass having good transmittance in a visible light range (380 nm to 770 nm) as a base and a rear substrate 20 to face each other and sealing a discharge medium such as Xe—Ne and Xe—He in a sealed space created by sealing the peripheral portions of the opposing surfaces of the front substrate 10 and rear substrate 20.
On a surface of the front substrate 10 facing the rear substrate 20, a pair of surface discharge display electrodes 11a and 11b extending in the first direction, X, are formed at a predetermined pitch, and a dielectric layer 12 for AC drive and a protecting layer 13 made of MgO are formed one upon another to cover the display electrodes 11a and 11b. In general, each of the display electrodes 11a and 11b is composed of a transparent electrode 14 such as ITO, and a bus electrode 15 made of a metal electrode material such a thick film of Ag. The bus electrode 15 has the function of decreasing the line resistance as well as the function of supplying a voltage to the transparent electrode 14 from an external circuit mounted outside the panel, and one end of the bus electrode 15 is guided to the peripheral portion of the front substrate 10, i.e., the edge of the front substrate 10 in one direction. The protecting layer 13 performs an important role to prevent ion impact on the dielectric layer 12 and emit secondary electrons for discharge.
On the other hand, a plurality of partition walls 21 extending in the second direction, Y, orthogonal to the first direction, X, are formed at a predetermined pitch on a surface of the rear substrate 20 facing the front substrate 10. Further, address electrodes 22 for address discharge are formed parallel to the partition walls 21 on the bottom face of grooves between the partition walls 21; a dielectric layer 23 is formed to cover the address electrodes 22; and fluorescence layers 24a, 24b and 24c of three colors, R, G and B, for color display are formed on the side faces of the partition walls 21 and the front face of the dielectric layer 23. A space enclosed by adjacent partition walls 21 within the sealed space is a discharge space. Note that the address electrode 22 is made of a metal electrode material such as Cr, Cu, and a thick film of Ag, performs the function of supplying a voltage from the external circuit mounted outside the panel, and is extended to the peripheral portion of the rear substrate 20.
Each region separated by the intersection of the display electrodes 11a, 11b and address electrodes 22 is the display area of a pixel, and selectively causes an address discharge for display writing by applying a voltage between one of the display electrode 11a (or 11b) and the address electrode 22, subsequently causes a discharge for maintaining the display in the cell where the address discharge is caused by applying a voltage between a pair of display electrodes 11a and 11b, and emits vacuum ultraviolet light when electrons collide with Xe in the discharge medium. The vacuum ultraviolet light is excited with visible light by the fluorescent layers 24a, 24b and 24c provided on the rear substrate 20, and the visible light is emitted to the outside.
Next, the following description will explain a general manufacturing method of a PDP which is the mainstream nowadays, and, here, a manufacturing method of the rear substrate 20 relating to the present invention.
(Electrode Formation Step)
First, after depositing a Cr/Cu/Cr metal thin film on a surface of a glass substrate by sputtering, address electrodes are formed in a desired pattern (for example, a pattern of straight lines) by patterning the metal thin film by a photolithography technique. It may, of course, be possible to form address electrodes in a desired pattern by depositing a photosensitive metal (for example, a photosensitive Ag paste) and then directly exposing the photosensitive metal.
(Dielectric Layer Formation Step)
Next, a dielectric layer is formed by applying a low-melting-point glass paste such as PbO—B2O3—ZnO-based glass material to the substrate by screen printing or roll coating and then sintering the applied low-melting-point glass paste at a desired sintering temperature (500 to 600° C.: about 15 minutes).
(Partition Wall Formation Step)
A low-melting-point glass paste to be partition walls is applied to the surface of the substrate by roll coating or other method, and then dried. Consequently, a partition wall material layer made of the low-melting-point glass is formed on the surface of the substrate. Next, a photosensitive resin film such as a dry film resist is attached to the surface of the substrate, and then the attached photosensitive resin film is formed into a mask pattern corresponding to the partition wall shape by a photolithography technique. The formed mask pattern makes an anti-sandblasting mask to be described later. Non-sintered partition walls having the shape of the mask pattern are formed by injecting an abrasive material such as glass beads and calcium carbonate with higher hardness than the partition wall material layer onto the surface of the substrate by sandblasting and cutting the partition wall material layer in regions other than the mask pattern. Then, after separating the photosensitive resin film from the substrate, the non-sintered partition walls are sintered to make glass under desired conditions (temperature: 500 to 600° C., and time: about 15 minutes), thereby completing the partition walls.
In the above-described manufacturing method, the partition wall material layer is necessary to make the partition walls though most part of the partition wall material layer is cut off by sandblasting, and therefore an increase in the cost is unavoidable. Moreover, since the partition wall material is cut by sandblasting before sintering, there is a possibility that foreign matter such as broken pieces of the partition walls may be produced during the partition wall formation step, and the produced foreign matter causes the problem of lower manufacturing yield.
Hence, in recent years, research has been conducted actively on a method of forming partition walls by direct carving of glass in which the partition walls are formed by directly cutting the glass substrate itself by sandblasting (for example, Japanese Patent Application Laid-Open No. 2001-43793). The following description explains a general method for forming partition walls by direct carving of glass that is currently under research.
(Partition Wall Formation Step)
First, a photosensitive resin film such as a dry film resist having resistance to sandblasting is attached to a surface of a glass substrate, and then the attached photosensitive resin film is formed into a desired mask pattern by a photolithography technique. The glass substrate in regions other than the mask pattern is cut off (depth of cutting: about 150 to 200 μm) by injecting an abrasive (particle diameter of about 10 to 20 μm) such as alumina and SiC with higher hardness than the glass substrate onto the surface of the glass substrate by sandblasting.
(Electrode Formation Step)
Next, after separating the photosensitive resin film from the substrate, a Cr/Cu/Cr metal thin film is deposited on the substrate surface by sputtering, and then a resist is applied to the surface of the substrate and dried. Thereafter, the resist in regions other than a region that is intended to be left as an electrode pattern is exposed and developed. An electrode having the electrode pattern is formed by removing an unnecessary metal thin film by etching.
(Dielectric Layer Formation Step)
Next, a dielectric layer is formed by applying a low-melting-point glass paste such as PbO—B2O3—ZnO-based glass material between the partition walls of the substrate by screen printing or other method and sintering the applied low-melting-point glass paste under desired sintering conditions (temperature: 500 to 600° C., and time: about 15 minutes).
In the above-described method of forming partition walls by direct carving of glass, since the partition walls are formed by directly cutting the glass substrate itself by sandblasting, there is no need to form the partition wall material layer, and thus it is possible to reduce the cost from the point of view of the material and processing step. Moreover, since the partition walls are formed by only processing the glass substrate, even when foreign matter such as broken pieces of the partition walls are produced, the number of choices for washing methods is increased because there is no possibility that the substrate is badly affected even if jet washing or ultrasonic washing, for example, is performed, and consequently the produced foreign matter can be easily removed. In other words, the method of removing foreign matter after the formation of the electrode and dielectric layer may have bad effects on the electrode and dielectric layer, whereas in the method of forming partition walls by direct carving of glass, since the removal of foreign matter is performed before the formation of the electrode and dielectric layer, there is no such bad effects.
However, during the formation of the electrode by the method of forming partition walls by direct carving of glass, since the partition walls have been formed when applying a resist to the substrate, there is a problem that air bubbles tend to be generated in the resist because the partition walls function as projections. Moreover, since the cross sectional configuration of the substrate is not uniform, the surface tension applied to the resist is not uniform, and the resist tends to gather in a region with a small radius of curvature (bent portion). As a result, the film thickness of the resist in the bent portion becomes thicker, while the film thickness of the resist in a region with a large radius of curvature becomes thinner, resulting in a problem of non-uniform film thickness of the resist. Thus, the etching shape, namely, electrode shape, varies when etching the metal thin film, and in the worst case, the electrode is disconnected, which causes the problem of lower manufacturing yield.
In addition, there is a proposed method in which, after forming partition walls by cutting a glass substrate, a conductive material is directly formed on a desired position by an ink-jet method. In this method, however, in order to ensure a film thickness required for the electrode, it is necessary to repeatedly draw the conductive material, and consequently the time required for electrode formation (tact time) becomes very long. Moreover, in order to form the shape and reduce the resistance, it is necessary to draw a large amount of electrode material, and consequently the cost is increased.